feat: add num_attempts to merge insert result (#2795)

This pipes the num_attempts field from lance's merge insert result
through lancedb. This allows callers of merge_insert to get a better
idea of whether transaction conflicts are occurring.

python/python/lancedb/_lancedb.pyi

@@ -306,6 +306,7 @@ class MergeResult:
     num_updated_rows: int
     num_inserted_rows: int
     num_deleted_rows: int
+    num_attempts: int
 
 class AddColumnsResult:
     version: int